1. Field of the Invention
The present invention relates to a filter arragement and in particular to a resonator band-pass filter arrangement for balanced and unbalanced line systems.
2. Description of the Prior Art
Band-pass filters are needed in almost any microwave application. In particular narrow-band transmit/receive circuits (so called transceiver-circuits), as they are used in mobile radio communication systems, need band-pass filters in order to suppress all interference signals from outside the used frequency band. Thus, large interference signals cannot limit the receiver and small interference signals do not deteriorate the basic noise. Further, band-pass filters are used in so-called mutiband systems in order to select the individual bands over frequency-separating filters, so-called diplexers.
Good band-pass filters distinguish themselves by showing very little electrical losses in the passband and having as much isolation as possible in the stopband and/or blocking band.
Frequently, resonators are used in band-pass filters. A serial resonator is concerned, if this two-terminal element shows a very good passband performance with the resonance frequency, if connected in series. For other frequencies this element shows a blocking performance. A parallel resonator is concerned, if this two-terminal element shows a very good passband performance with the resonance frequency if connected between the signal path and a reference potential, like e.g. ground. For other frequencies this element shows a blocking performance.
A balanced transformer (Balun) is used, where a transition from an unbalanced to a balanced line system, e.g. a microwave line system, or the contrary is necessary. In modern handset devices small signal transmitter/receiver circuits (small signal transceiver) integrated in semiconductors exclusively have balanced inputs and outputs. However, power amplifiers and antennas are implemented in an unbalanced technique. Therefore, two networks are required for every frequency band in such handset devices in order to realize low-loss transitions between the balanced power system and the unbalanced power system. In the mobile radio area unbalanced circuits, like e.g. diplexers, are the conventional and only used circuits so far. Further, a large number of line systems used in practice are unbalanced systems, like e.g. coaxial, microstrip and strip lines.
For realizing band-pass filters a large numbers of realization possibilities exist.
For many decades filters having concentrated inductive members and capacities or having line structures have been known. In frequency-separating filters (diplexers) usually pure line structures are used.
For narrow-band filter applications surface acoustic wave filters, the so-called SAW-filters, have been used for years, in which non-coupled resonators are used. Such non-coupled resonators are for example arranged in the so-called ladder-type structure.
For narrow-band band-pass filters realizations with coupled resonators have only become established for circuits in the mobile radio area in the last few years. These filter realizations especially distinguish themselves by their very steep filter edges. This technology finds its use with large resonators, e.g. of cylindrical ceramics with low losses and a large dielectric constant and with filters for great powers, like they are for example used in base stations.
The above described, known filter realizations are disadvantageous in so far, that they are exclusively available for balanced line systems or exclusively for unbalanced line systems. Thus, the filter realizations either comprise a balanced input gate and a balanced output gate or an unbalanced input gate and an unbalanced output gate. Mixed constructions of coupled resonators and concentrated devices, like for example inductive devices, are not known.
Omitting the balanced transformer, the balun, is only known in the art in combination with surface acoustic wave filters (SAW filters). The relatively extensive and costly SAW filters offer a possibility of balancing the unbalanced signals by inserting a physically very short 180° degree line in the acoustic area. SAW filters offer a very steep filter characteristic with average throughput losses of about 2 to 3 dB and have established themselves as receiving filters ahead of the preamplifiers of the transmit-receive circuits. On the transmission side of the transmit/receive circuits mainly balanced transformers (Baluns) realized in ceramics are used.
The drawback of the above-described realizations of band-pass filters known in the art is on the one hand, that band-pass filters having discrete inductive devices and capacitors for commercial microwave products produced in large amounts are of no interest, as the number of required devices is very high and therefore the required area is very large. Further, the losses in the passband are intolerably high. In contrast to that, coupled resonator filters offer better selection properties with lower losses than the “interconnected” resonator filters, like for example the ladder type structures. The commercially very successful SAW filters are based on an antiquated topology due to their non-coupled resonator technology.
For realizing frequency-separating filters, diplexers, the SAW filters comprise a too high passband attenuation, which lies at about 0,7 dB with diplexers. Further, SAW filters are not large-signal stable, so that the signal output by a power amplifier would destroy the filter. After the output of the transmit/receive circuit a SAW filter is too expensive.
A balanced transformer has too little selection properties compared to a SAW filter, so that its use is not advantageous either.
Regarding the realizations for band-pass filters known in the art it can be summarised, that they are too expensive to realize, comprise a too high loss in the passband and/or a too low selection property.
U.S. Pat. No. 1,848,221 describes a filter circuit that consists of a serial inductivity, from a series coil and from two parallel branches, which respectively consist of a serial circuit, from an inductivity and a capacitor. The filter is connected between an input circuit and an output circuit, wherein the coils are coupled to each other in the parallel branches.
JP 613706A describes a band-pass filter which comprises an LC series resonance circuit which is connected in series with another LC series resonance circuit. An LC parallel resonance circuit is connected between the connection of the two LC series resonance circuits and ground. The inductivities used in the LC series resonance circuits are coupled to each other.
In the article by Orlov A. T., “Use of active Networks to widen the spectrum of application of piezoelectric filters”, in “Frequency Control Symposium, 1994, Proceedings of the 1994 IEEE. International Boston, Jun. 1, 1994, pages 411 to 414” the design of active BAW piezoelectric filters is described whose properties are not obtainable with inductivity-free passive piezoelectric filters.